RESUMO
Gene expression is adjusted according to cellular needs through a combination of mechanisms acting at different layers of the flow of genetic information. At the posttranscriptional level, RNA-binding proteins are key factors controlling the fate of nascent and mature mRNAs. Among them, the members of the CsrA family are small dimeric proteins with heterogeneous distribution across the bacterial tree of life, that act as global regulators of gene expression because they recognize characteristic sequence/structural motifs (short hairpins with GGA triplets in the loop) present in hundreds of mRNAs. The regulatory output of CsrA binding to mRNAs is counteracted in most cases by molecular mimic, non-protein coding RNAs that titrate the CsrA dimers away from the target mRNAs. In γ-proteobacteria, the regulatory modules composed by CsrA homologs and the corresponding antagonistic sRNAs, are mastered by two-component systems of the GacS-GacA type, which control the transcription and the abundance of the sRNAs, thus constituting the rather linear cascade Gac-Rsm that responds to environmental or cellular signals to adjust and coordinate the expression of a set of target genes posttranscriptionally. Within the γ-proteobacteria, the genus Pseudomonas has been shown to contain species with different number of active CsrA (RsmA) homologs and of molecular mimic sRNAs. Here, with the help of the increasing availability of genomic data we provide a comprehensive state-of-the-art picture of the remarkable multiplicity of CsrA lineages, including novel yet uncharacterized paralogues, and discuss evolutionary aspects of the CsrA subfamilies of the genus Pseudomonas, and implications of the striking presence of csrA alleles in natural mobile genetic elements (phages and plasmids).
RESUMO
In bacteria, the 5'-end-dependent RNA degradation is triggered by the RNA pyrophosphohydrolase RppH converting tri/diphosphate to monophosphate transcripts. This study shows that in the soil bacterium Azotobacter vinelandii, inactivation of rppH gene negatively affected the production of bioplastic poly-ß-hydroxybutyrate (PHB) by reducing the expression at the translational level of PhbR, the specific transcriptional activator of the phbBAC biosynthetic operon. The effect of RppH on the translation of phbR seemed to be exerted through the translational repressor RsmA, as the inactivation of rsmA in the rppH mutant restored the phbR expression. Interestingly, in Escherichia coli inactivation of rppH also affected the expression of CsrA, the RsmA homolog. The level of the csrA transcript was higher and more stable in the E. coli rppH mutant than in the wild type strain. Additionally, and in contrast to the csrA mutants that are known to have a defective swimming phenotype, the E. coli rppH mutant showed a hyper-swimming phenotype that was suppressed by a csrA mutation, and the AvRppH restored to wild type level the swimming phenotype to the E. coli rppH mutant. We propose that in both A. vinelandii and E. coli, RppH activity plays a role in the expression of the translational regulator protein RsmA/CsrA.